A planar space frame is generally defined as a truss-like structure system in the form of a network of elements (as opposed to a continuous surface) so arranged that forces are transferred in a three-dimensional manner. A space frame is typically constructed from interlocking struts or members in a geometric pattern. Macroscopically, a planar space frame takes the form of a flat or generally flat structure (slight curvature and/or slight change in thickness may be present).
Space frames have been largely used in architecture and civil engineering to build large structures. They are recognized and used for their mechanical properties, and particularly their rigidity, stiffness and ability to resist heavy concentrated load, as well as for their lightness, and their economy in use of materials.
Planar space frame structures have not been used to the same extent in automotive engineering because of the much smaller scale, the complexity of defining economically viable fabrication methods, and the lack of appropriate usages.
U.S. Pat. No. 6,207,256 to Tashiro introduced various methods to produce some embodiments of a planar space frame (space truss) composite panel of few tens of centimeters in thickness, or less. Tashiro's concepts using sheet metal still present some drawbacks in terms of manufacturability, design flexibility and mechanical properties for chassis or unibody constructions. Among other things, the proposed embodiments do not efficiently distribute sheer and torsional stresses which are important in a body construction of commercial vehicles, and they do not easily allow for a variation in width of the ribs for specific local load requirements and/or global load paths.
As know from the art, commercial vehicles such as city buses generally have a body frame integral construction also known as a unibody construction. In such a construction, a load bearing lattice structure comprises longitudinal and transverse horizontal beams and vertical uprights attached to each other in intersections. The integral frame is subsequently covered with sheet cladding materials riveted, glued or otherwise fixed. The exterior cladding is generally sheet aluminum, stainless steel or molded fiberglass skins. Floor panels are generally fabricated from plywood and composite materials.
Roof and floor panels may also present a sandwich-like construction in which two load bearing skins are separated by a core of stiffening lightweight material to provide higher strength and rigidity in some areas. Structural sandwich constructions with complex cores such as honeycomb cores are not intensively used in the automotive industry mainly due to their high price. Simple corrugated cores between two flat sheets are more common.
With a typical unibody construction, the body interior of a mass transit vehicle generally includes recesses for front and rear wheels and other vehicle components such as batteries and controllers in many battery-dominant electric buses. On the roof, we generally found enclosures for components such as HVAC and energy storage equipments (batteries, power electronics controllers, charging equipments, gas cylinders, etc.). These constructions have known disadvantages. Recesses inside the vehicle affect passenger capacity, flexibility in seating layout, passenger flow, and create steps or higher floor section inside the vehicle. Storing components on the roof raises the center of mass affecting the vehicle dynamics such as its tendency to roll, and generates higher stresses on the body construction. This impacts the overall structure which needs to be reinforced, which generally contribute to increase the overall vehicle curb weight and the manufacturing cost. These drawbacks are particularly visible in a majority of new battery-dominant electric buses having a large amount of batteries.
The trend toward function integration and lightweight design favor the reengineering of existing systems and components in order to permit an optimal usage of material, to reduce part count and assembly complexity, and to reduce vehicle weight. Accordingly, there is a need for an improved floor/roof panel construction providing both structural properties and the capability of housing embedded systems.
While prior vehicle constructions have been adequate for many purposes, they have lacked some of the advantages obtainable with the embodiments of the present invention. Many advantages of the invention are clearly described hereinafter and others shall be readily apparent to those skilled in the art.